The invention relates to an imaging apparatus comprising an imaging system and an opto-electronic focus detection system for determining a deviation between the image plane of the imaging system and a second plane on which imaging is to take place, said focus detection system comprising a radiation source for supplying a focus detection beam, a radiation-sensitive detector arranged at the same side of the second plane as the radiation source, and optical elements for directing the focus detection beam onto the second plane at a small angle to said plane for changing the vergence of said beam and of that of the beam reflected by the second plane and for directing the last-mentioned beam onto the detector.
An imaging apparatus of this type is known from, for example, U.S. Pat. No. 4,866,262. The apparatus described in this previous Patent may be part of what is commonly referred to as "wafer stepper", i.e. a machine for repetitively imaging a mask pattern on a substrate, for example, a silicon substrate by means of optical radiation, which machine is used in the manufacture of, for example, integrated circuits, briefly referred to as ICs. The pattern is imaged on a first sub-area, or field, of the substrate by means of a projection lens system and subsequently the substrate is moved over an accurately defined distance whereafter the pattern is imaged on a second substrate field, whereafter the substrate is moved again, and so forth until the pattern is imaged on all substrate fields.
Due to the fineness of the details which must be imaged, the projection lens system should have a large numerical aperture. However, such a lens system has a small depth of focus so that a deviation between the image plane of the projection lens system and the substrate surface must be accurately measured and possibly corrected. Said measurement is generally referred to as focus detection. Another problem is that the substrate surface may be arranged obliquely due to tilt of the substrate as a whole or thickness variations of this substrate. Moreover, the substrate surface may have local unevennesses. It is therefore necessary to perform a focus detection for each substrate field.
In the apparatus described in U.S. Pat. No. 4,866,262 the focus detection is performed with the aid of a laser beam which is incident on the substrate surface at a large angle, for example, of the order of 80.degree., to the normal on this surface where it forms a first radiation spot. The substrate surface reflects the beam towards a radiation-sensitive detector comprising two detector cells and forms a second radiation spot in the plane of the detector, which spot is an image of the frist radiation spot. If the substrate surface coincides with the image plane of the projection lens system, the second radiation spot is located symmetrically with respect to the detector cells and the difference between the output signals of the detector cells is equal to zero. If the substrate surface does not coincide with said image plane, the second radiation spot is not located symmetrically with respect to the detector cells and said difference signal is unequal to zero. The magnitude and direction of a deviation between the substrate surface and the image plane of the projection lens system can be derived from the amplitude and the sign of this difference signal.